Method and apparatus for cancelling interference and receiving signal in wireless communication system

ABSTRACT

The present invention relates to a wireless communication system. A method for cancelling interference and receiving a signal by a user equipment in a wireless communication system, the method performed by the user equipment comprising: receiving assistance information for cancelling an interference signal transmitted from an interfering base station; cancelling the interference signal based on the assistance information; and receiving a desired signal from a serving base station, wherein the user equipment assumes a part of the assistance information for cancelling the interference signal as a limited value and then receives the interference signal.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method for cancelling interference and receivinga signal in a wireless communication system and an apparatus forsupporting the same.

BACKGROUND ART

Multiple input multiple output (MIMO) increases the efficiency of datatransmission and reception using multiple transmit antennas and multiplereceive antennas instead of a single transmission antenna and a singlereception antenna. A receiver receives data through multiple paths whenmultiple antennas are used, whereas the receiver receives data through asingle antenna path when a single antenna is used. Accordingly, MIMO canincrease a data transmission rate and throughput and improve coverage.

A single cell MIMO scheme can be classified into a single user-MIMO(SU-MIMO) scheme for receiving a downlink signal by a single UE in onecell and a multi user-MIMO (MU-MIMO) scheme for receiving a downlinksignal by two or more UEs.

Channel estimation refers to a procedure for compensating for signaldistortion due to fading to restore a reception signal. Here, the fadingrefers to sudden fluctuation in signal intensity due to multipath-timedelay in a wireless communication system environment. For channelestimation, a reference signal (RS) known to both a transmitter and areceiver is required. In addition, the RS can be referred to as a RS ora pilot signal according to applied standard.

A downlink RS is a pilot signal for coherent demodulation for a physicaldownlink shared channel (PDSCH), a physical control format indicatorchannel (PCFICH), a physical hybrid indicator channel (PHICH), aphysical downlink control channel (PDCCH), etc. A downlink RS includes acommon RS (CRS) shared by all user equipments (UEs) in a cell and adedicated RS (DRS) for a specific UE. For a system (e.g., a systemhaving extended antenna configuration LTE-A standard for supporting 8transmission antennas) compared with a conventional communication system(e.g., a system according to LTE release-8 or 9) for supporting 4transmission antennas, DRS based data demodulation has been consideredfor effectively managing RSs and supporting a developed transmissionscheme. That is, for supporting data transmission through extendedantennas, DRS for two or more layers can be defined. DRS is pre-coded bythe same pre-coder as a pre-coder for data and thus a receiver caneasily estimate channel information for data demodulation withoutseparate precoding information.

A downlink receiver can acquire pre-coded channel information forextended antenna configuration through DRS but requires a separate RSother than DRS in order to non-pre-coded channel information.Accordingly, a receiver of a system according to LTE-A standard candefine a RS for acquisition of channel state information (CSI), that is,CSI-RS.

DISCLOSURE Technical Problem

Based on the aforementioned discussion, an object of the presentinvention is to provide a method and device for transmitting andreceiving channel state information in a wireless communication system.

The technical objects that can be achieved through the present inventionare not limited to what has been particularly described hereinabove andother technical objects not described herein will be more clearlyunderstood by persons skilled in the art from the following detaileddescription.

Technical Solution

To solve the aforementioned problem, according to one embodiment of thepresent invention, a method for enabling a user equipment to cancelinterference and receive a signal in a wireless communication systemcomprises the steps of receiving assistance information for cancellingan interference signal transmitted from an interfering base station; andcancelling the interference signal on the basis of the assistanceinformation and receiving a desired signal from a serving base station,wherein the user equipment assumes a part of the assistance informationfor cancelling the interference signal as a limited value and thenreceives the interference signal.

According to another embodiment of the present invention, a userequipment for cancelling interference and receiving data in a wirelesscommunication system comprises a radio frequency (RF) unit; and aprocessor, wherein the processor is configured to receive assistanceinformation for cancelling an interference signal transmitted from aninterfering base station, and cancel the interference signal on thebasis of the assistance information and receive a desired signal from aserving base station, and the user equipment assumes a part of theassistance information for cancelling the interference signal as alimited value and then receives the interference signal.

The followings may commonly be applied to the embodiments of the presentinvention.

The user equipment may assume that the number of data layers of theinterference signal is smaller than or equal to the number of datalayers of the desired signal.

The user equipment may assume that the number of data layers of theinterference signal is smaller than or equal to a value obtained bysubtracting the number of data layers of the desired signal from thenumber of receiving antennas of the user equipment.

The user equipment may assume that a modulation order of theinterference signal is smaller than or equal to a value obtained bysubtracting the number of data layers of the desired signal from thenumber of receiving antennas of the user equipment.

The user equipment may further calculate PMI of the interfering basestation on the basis of the number of data layers of the assumedinterference signal.

The user equipment may further receive whether a starting symbol of aninterference PDSCH (Physical Downlink Control Channel) can be calculatedthrough a PCFCH (Physical Control Format Indicator Channel) of theinterfering base station, through RRC (Radio Resource Control)signaling.

The user equipment may further receive information on an interferenceDM-RS (demodulation reference signal), which may be connected with a CRS(Common Reference Signal) of the interfering base station through QCL(Quasi Co-location) A.

It will be appreciated by persons skilled in the art that that theeffects that could be achieved with the present invention are notlimited to what has been particularly described hereinabove and otheradvantages of the present invention will be more clearly understood fromthe following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a diagram illustrating an example of a structure of a downlinkradio frame;

FIG. 2 is a diagram illustrating an example of a resource grid for onedownlink slot;

FIG. 3 is a diagram illustrating a structure of a downlink subframe;

FIG. 4 is a diagram illustrating a structure of an uplink subframe;

FIG. 5 is a schematic diagram illustrating a wireless communicationsystem having multiple antennas;

FIG. 6 is a diagram illustrating legacy CRS and DRS patterns;

FIG. 7 is a diagram illustrating an example of a DM RS pattern;

FIG. 8 is a diagram illustrating examples of a CSI-RS pattern;

FIG. 9 is a diagram illustrating an example of a method for periodicallytransmitting a CSI-RS;

FIG. 10 is a diagram illustrating an example of a method fornon-periodically transmitting a CSI-RS;

FIG. 11 is a diagram illustrating an example of two CSI-RSconfigurations which are used;

FIG. 12 is a diagram illustrating a general interference environment ofa downlink system;

FIG. 13 is a diagram illustrating an example of PDSCH interferencecancellation based on a PDSCH IC starting symbol according to thepresent invention;

FIG. 14 is a diagram illustrating an example of CRS assistanceinformation according to the present invention;

FIG. 15 is a diagram illustrating an example of DM-RS assistanceinformation according to the present invention;

FIG. 16 is a flow chart illustrating a method for receiving a signal,which may be applied to one embodiment of the present invention; and

FIG. 17 is a diagram illustrating a base station and a user equipmentwhich may be applied to one embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments described below correspond to predetermined combinationsof elements and features and characteristics of the present invention.Moreover, unless mentioned otherwise, the characteristics of the presentinvention may be considered as optional features of the presentinvention. Herein, each element or characteristic of the presentinvention may also be operated or performed without being combined withother elements or characteristics of the present invention.Alternatively, the embodiment of the present invention may be realizedby combining some of the elements and/or characteristics of the presentinvention. Additionally, the order of operations described according tothe embodiment of the present invention may be varied. Furthermore, partof the configuration or characteristics of any one specific embodimentof the present invention may also be included in (or shared by) anotherembodiment of the present invention, or part of the configuration orcharacteristics of any one embodiment of the present invention mayreplace the respective configuration or characteristics of anotherembodiment of the present invention.

In the description of the present invention, the embodiments of thepresent invention will be described by mainly focusing on the datatransmission and reception relation between a base station and a userequipment. Herein, the base station may refer to a terminal node of thenetwork that performs direct communication with the user equipment (oruser terminal). In the description of the present invention, particularoperations of the present invention that are described as beingperformed by the base station may also be performed by an upper node ofthe base station.

More specifically, in a network consisting of multiple network nodesincluding the base station, diverse operations that are performed inorder to communicate with the terminal (or user equipment) may beperformed by the base station or network nodes other than the basestation. Herein, the term ‘Base Station (BS)’ may be replaced by otherterms, such as fixed station, Node B, eNode B (eNB), ABS (Advanced BaseStation), or Access Point (AP). Relay may be replaced by other terms,such as Relay Node (RN), Relay Station (RS), and so on. Furthermore,‘Terminal’ may be replaced by other terms, such as UE (User Equipment),MS (Mobile Station), MSS (Mobile Subscriber Station), SS (SubscriberStation), and so on.

It should be noted that specific terms disclosed in the presentinvention are proposed for convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to other formats within the technical scope orspirit of the present invention.

In some instances, well-known structures and devices are omitted inorder to avoid obscuring the concepts of the present invention andimportant functions of the structures and devices are shown in blockdiagram form. The same reference numbers will be used throughout thedrawings to refer to the same or like parts.

Exemplary embodiments of the present invention are supported by standarddocuments disclosed for at least one of wireless access systemsincluding an institute of electrical and electronics engineers (IEEE)802 system, a 3rd generation partnership project (3GPP) system, a 3GPPlong term evolution (LTE) system, an LTE-advanced (LTE-A) system, and a3GPP2 system. In particular, steps or parts, which are not described toclearly reveal the technical idea of the present invention, in theembodiments of the present invention may be supported by the abovedocuments. All terminology used herein may be supported by at least oneof the above-mentioned documents.

The following embodiments of the present invention can be applied to avariety of wireless access technologies, for example, code divisionmultiple access (CDMA), frequency division multiple access (FDMA), timedivision multiple access (TDMA), orthogonal frequency division multipleaccess (OFDMA), single carrier frequency division multiple access(SC-FDMA), and the like. CDMA may be embodied through wireless (orradio) technology such as universal terrestrial radio access (UTRA) orCDMA2000. TDMA may be embodied through wireless (or radio) technologysuch as global system for mobile communication (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). OFDMAmay be embodied through wireless (or radio) technology such as instituteof electrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE802.16 (WiMAX), IEEE 802-20, and evolved UTRA (E-UTRA). UTRA is a partof universal mobile telecommunications system (UMTS). 3rd generationpartnership project (3GPP) long term evolution (LTE) is a part of E-UMTS(Evolved UMTS), which uses E-UTRA. 3GPP LTE employs OFDMA in downlinkand employs SC-FDMA in uplink. LTE—Advanced (LTE-A) is an evolvedversion of 3GPP LTE. WiMAX can be explained by IEEE 802.16e(wirelessMAN-OFDMA reference system) and advanced IEEE 802.16m(wirelessMAN-OFDMA advanced system). For clarity, the followingdescription focuses on IEEE 802.11 systems. However, technical featuresof the present invention are not limited thereto.

With reference to FIG. 1, the structure of a downlink radio frame willbe described below.

In a cellular orthogonal frequency division multiplexing (OFDM) wirelesspacket communication system, uplink and/or downlink data packets aretransmitted in subframes. One subframe is defined as a predeterminedtime period including a plurality of OFDM symbols. The 3GPP LTE standardsupports a type-1 radio frame structure applicable to frequency divisionduplex (FDD) and a type-2 radio frame structure applicable to timedivision duplex (TDD).

FIG. 1 illustrates the type-1 radio frame structure. A downlink radioframe is divided into 10 subframes. Each subframe is further dividedinto two slots in the time domain. A unit time during which one subframeis transmitted is defined as a Transmission Time Interval (TTI). Forexample, one subframe may be 1 ms in duration and one slot may be 0.5 msin duration. A slot includes a plurality of OFDM symbols in the timedomain and a plurality of resource blocks (RBs) in the frequency domain.Because the 3GPP LTE system adopts OFDMA for downlink, an OFDM symbolrepresents one symbol period. An OFDM symbol may be referred to as anSC-FDMA symbol or symbol period. An RB is a resource allocation unitincluding a plurality of contiguous subcarriers in a slot.

The number of OFDM symbols in one slot may vary depending on a cyclicprefix (CP) configuration. There are two types of CPs: extended CP andnormal CP. In the case of the normal CP, one slot includes 7 OFDMsymbols. In the case of the extended CP, the length of one OFDM symbolis increased and thus the number of OFDM symbols in a slot is smallerthan in the case of the normal CP. Thus when the extended CP is used,for example, 6 OFDM symbols may be included in one slot. If channelstate gets poor, for example, during fast movement of a UE, the extendedCP may be used to further decrease inter-symbol interference (ISI).

In the case of the normal CP, one subframe includes 14 OFDM symbolsbecause one slot includes 7 OFDM symbols. The first two or three OFDMsymbols of each subframe may be allocated to a physical downlink controlchannel (PDCCH) and the other OFDM symbols may be allocated to aphysical downlink shared channel (PDSCH).

The above-described radio frame structures are purely exemplary and thusit is to be noted that the number of subframes in a radio frame, thenumber of slots in a subframe, or the number of symbols in a slot mayvary.

FIG. 2 illustrates the structure of a downlink resource grid for theduration of one downlink slot. FIG. 2 corresponds to a case in which anOFDM includes normal CP. Referring to FIG. 2, a downlink slot includes aplurality of OFDM symbols in the time domain and includes a plurality ofRBs in the frequency domain. Here, one downlink slot includes 7 OFDMsymbols in the time domain and an RB includes 12 subcarriers in thefrequency domain, which does not limit the scope and spirit of thepresent invention. An element on a resource grid is referred to as aresource element (RE). For example, RE a(k,l) refers to RE location in akth subcarrier and a first OFDM symbol. In the case of the normal CP,one RB includes 12×7 REs (in the case of the extended CP, one RBincludes 12×6 REs). An interval between subcarriers is 15 kHz and thusone RB includes about 180 kHz in the frequency domain. NDL is number ofRBs in a downlink slot. NDL depends on a downlink transmission bandwidthconfigured by BS scheduling.

FIG. 3 illustrates the structure of a downlink subframe. Up to threeOFDM symbols at the start of the first slot in a downlink subframe areused for a control region to which control channels are allocated andthe other OFDM symbols of the downlink subframe are used for a dataregion to which a PDSCH is allocated. A basic unit of transmission isone subframe. That is, a PDCCH and a PDSCH are allocated across twoslots. Downlink control channels used in the 3GPP LTE system include,for example, a physical control format indicator channel (PCFICH), aphysical downlink control channel (PDCCH), and a physical hybridautomatic repeat request (HARQ) indicator channel (PHICH). The PCFICH islocated in the first OFDM symbol of a subframe, carrying informationabout the number of OFDM symbols used for transmission of controlchannels in the subframe. The PHICH delivers an HARQACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal in response toan uplink transmission. Control information carried on the PDCCH iscalled downlink control information (DCI). The DCI transports uplink ordownlink scheduling information, or uplink transmission power controlcommands for UE groups. The PDCCH delivers information about resourceallocation and a transport format for a downlink shared channel(DL-SCH), resource allocation information about an uplink shared channel(UL-SCH), paging information of a paging channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a random access responsetransmitted on the PDSCH, a set of transmission power control commandsfor individual UEs of a UE group, transmission power controlinformation, voice over Internet protocol (VoIP) activation information,etc. A plurality of PDCCHs may be transmitted in the control region. AUE may monitor a plurality of PDCCHs. A PDCCH is formed by aggregatingone or more consecutive Control Channel Elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel. A CCE corresponds to a plurality of REgroups. The format of a PDCCH and the number of available bits for thePDCCH are determined according to the correlation between the number ofCCEs and a coding rate provided by the CCEs. An eNB determines the PDCCHformat according to DCI transmitted to a UE and adds a cyclic redundancycheck (CRC) to control information. The CRC is masked by an identifier(ID) known as a radio network temporary identifier (RNTI) according tothe owner or usage of the PDCCH. When the PDCCH is directed to aspecific UE, its CRC may be masked by a cell-RNTI (C-RNTI) of the UE.When the PDCCH is for a paging message, the CRC of the PDCCH may bemasked by a paging indicator identifier (P-RNTI). When the PDCCH carriessystem information, particularly, a system information block (SIB), itsCRC may be masked by a system information ID and a system informationRNTI (SI-RNTI). To indicate that the PDCCH carries a random accessresponse in response to a random access preamble transmitted by a UE,its CRC may be masked by a random access-RNTI (RA-RNTI).

FIG. 4 illustrates the structure of an uplink subframe. An uplinksubframe may be divided into a control region and a data region in thefrequency domain. A Physical Uplink Control Channel (PUCCH) carryinguplink control information is allocated to the control region and aphysical uplink shared channel (PUSCH) carrying user data is allocatedto the data region. To maintain the property of a single carrier, a UEdoes not transmit a PUSCH and a PUCCH simultaneously. A PUCCH for a UEis allocated to an RB pair in a subframe. The RBs of the RB pair occupydifferent subcarriers in two slots. Thus it is said that the RB pairallocated to the PUCCH is frequency-hopped over a slot boundary.

Modeling of MIMO System

A multiple input multiple output (MIMO) system increasestransmission/reception efficiency of data using multiple transmission(Tx) antennas and multiple reception (Rx) antennas. MIMO technology doesnot depend upon a single antenna path in order to receive all messagesbut instead can combine a plurality of data fragments received through aplurality of antennas and receive all data.

MIMO technology includes a spatial diversity scheme, a spatialmultiplexing scheme, etc. The spatial diversity scheme can increasetransmission reliability or can widen a cell diameter with diversitygain and thus is appropriate for data transmission of a UE that moves ahigh speed. The spatial multiplexing scheme can simultaneously transmitdifferent data so as to increase data transmission rate without increasein a system bandwidth.

FIG. 5 illustrates the configuration of a MIMO communication systemhaving multiple antennas. As illustrated in FIG. 5(a), the simultaneoususe of a plurality of antennas at both the transmitter and the receiverincreases a theoretical channel transmission capacity, compared to useof a plurality of antennas at only one of the transmitter and thereceiver. Therefore, transmission rate may be increased and frequencyefficiency may be remarkably increased. As channel transmission rate isincreased, transmission rate may be increased, in theory, to the productof a maximum transmission rate Ro that may be achieved with a singleantenna and a transmission rate increase Ri.

R _(i)=min(N _(T) , N _(R))   [Equation 1]

For instance, a MIMO communication system with four Tx antennas and fourRx antennas may achieve a four-fold increase in transmission ratetheoretically, relative to a single-antenna system. Since thetheoretical capacity increase of the MIMO system was verified in themiddle 1990s, many techniques have been actively proposed to increasedata rate in real implementation. Some of the techniques have alreadybeen reflected in various wireless communication standards for 3G mobilecommunications, future-generation wireless local area network (WLAN),etc.

Concerning the research trend of MIMO up to now, active studies areunderway in many respects of MIMO, inclusive of studies of informationtheory related to calculation of multi-antenna communication capacity indiverse channel environments and multiple access environments, studiesof measuring MIMO radio channels and MIMO modeling, studies oftime-space signal processing techniques to increase transmissionreliability and transmission rate, etc.

Communication in a MIMO system will be described in detail throughmathematical modeling. It is assumed that NT Tx antennas and NR Rxantennas are present in the system.

Regarding a transmission signal, up to NT pieces of information can betransmitted through the NT Tx antennas, as expressed in Equation 2below.

s=└s₁, s₂, . . . , s_(N) _(T) ┘^(T)   [Equation 2]

A different transmission power may be applied to each piece oftransmission information, s₁, s₂, . . . , s_(N) _(T) . Let thetransmission power levels of the transmission information be denoted byP₁, P₂, . . . , P_(N) _(T) , respectively. Then the transmissionpower-controlled transmission information vector is given as

ŝ=[ŝ₁, ŝ₂, . . . , ŝ_(N) _(T) ]^(T)=[P₁s₁, P₂s₂, . . . , P_(N) _(T)s_(N) _(T) ]^(T)   [Equation 3]

The transmission power-controlled transmission information vector ŝ maybe expressed as follows, using a diagonal matrix P of transmissionpower.

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {Ps}}} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

NT transmission signals x₁, x₂, . . . , x_(N) _(T) may be generated bymultiplying the transmission power-controlled information vector ŝ by aweight matrix W. The weight matrix W functions to appropriatelydistribute the transmission information to the Tx antennas according totransmission channel states, etc. These NT transmission signals x₁, x₂,. . . , x_(N) _(T) are represented as a vector x, which may bedetermined by Equation 5 below.

$\begin{matrix}\begin{matrix}{x = \begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix}} \\{= {\begin{bmatrix}w_{11} & w_{12} & \ldots & w_{1N_{T}} \\w_{21} & w_{22} & \ldots & w_{2N_{T}} \\\vdots & \; & \ddots & \; \\w_{i\; 1} & w_{i\; 2} & \ldots & w_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\w_{N_{T}1} & w_{N_{T}2} & \ldots & w_{N_{T}N_{T}}\end{bmatrix}\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}}} \\{= {{W\hat{s}} = {WPs}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Here, W_(ij) refers to a weight between an ith Tx antenna and jthinformation.

A reception signal x may be considered in different ways according totwo cases (e.g., spatial diversity and spatial multiplexing). In thecase of spatial multiplexing, different signals are multiplexed and themultiplexed signals are transmitted to a receiver, and thus, elements ofinformation vector (s) have different values. In the case of spatialdiversity, the same signal is repeatedly transmitted through a pluralityof channel paths and thus elements of information vectors (s) have thesame value. A hybrid scheme of spatial multiplexing and spatialdiversity can also be considered. That is, that same signal may betransmitted through three Tx antennas and the remaining signals may bespatial-multiplexed and transmitted to a receiver.

In the case of NR Rx antennas, a receiption signal of each antenna maybe expressed as the vector shown in Equation 6 below.

y=[y₁, y₂, . . . , y_(N) _(R) ]^(T)   [Equation 6]

When a channel modeling is executed in the MIMO communication system,individual channels can be distinguished from each other according totransmission/reception (Tx/Rx) antenna indexes. A channel passing therange from a Tx antenna j to an Rx antenna i is denoted by hij. Itshould be noted that the index order of the channel hij is locatedbefore a reception (Rx) antenna index and is located after atransmission (Tx) antenna index.

FIG. 5(b) illustrates channels from NT Tx antennas to an Rx antenna i.The channels may be collectively represented in the form of vector andmatrix. Referring to FIG. 5(b), the channels passing the range from theNT Tx antennas to the Rx antenna i can be represented by the Equation 7below.

h_(i) ^(T)=[h_(i1), h_(i2), . . . , h_(iN) _(T) ]  [Equation 7]

All channels passing the range from the NT Tx antennas to NR Rx antennasare denoted by the matrix shown in Equation 8 below.

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{r}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}}} & \left\lbrack {{Equation}\mspace{14mu} 8} \right\rbrack\end{matrix}$

Additive white Gaussian noise (AWGN) is added to an actual channel whichhas passed the channel matrix. The AWGN (n1, n2, . . . , nNR) added toeach of NR reception (Rx) antennas can be represented by Equation 9below.

n=[n₁, n₂, . . . , n_(N) _(R) ]^(T)   [Equation 9]

A reception signal calculated by the above-mentioned equations can berepresented by Equation 10 below.

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\begin{bmatrix}h_{11} & h_{12} & \ldots & h_{1N_{T}} \\h_{21} & h_{22} & \ldots & h_{2N_{T}} \\\vdots & \; & \ddots & \; \\h_{i\; 1} & h_{i\; 2} & \ldots & h_{{iN}_{T}} \\\vdots & \; & \ddots & \; \\h_{N_{R}1} & h_{N_{R}2} & \ldots & h_{N_{R}N_{T}}\end{bmatrix}\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \left\lbrack {{Equa}\; {tion}\mspace{14mu} 10} \right\rbrack\end{matrix}$

The number of rows and the number of columns of a channel matrix Hindicating a channel condition are determined by the number of Tx/Rxantennas. In the channel matrix H, the number of rows is equal to thenumber (NR) of Rx antennas, and the number of columns is equal to thenumber (NT) of Tx antennas. Namely, the channel matrix H is denoted byan NR×NT matrix.

The rank of a matrix is defined as the smaller between the number ofindependent rows and the number of independent columns in the channelmatrix. Accordingly, the rank of the channel matrix is not larger thanthe number of rows or columns of the channel matrix. The rank of achannel matrix H, rank(H) satisfies the following constraint.

rank(H)≦min(N _(T) , N _(R))   [Equation 11]

For MIMO transmission, ‘rank’ indicates the number of paths forindependent transmission of signals and ‘number of layers’ indicates thenumber of streams transmitted through each path. In general, atransmission end transmits layers, the number of which corresponds tothe number of ranks used for signal transmission, and thus, rank havethe same meaning as number of layers unless there is no differentdisclosure.

Reference Signals (RSs)

In a wireless communication system, a packet is transmitted on a radiochannel. In view of the nature of the radio channel, the packet may bedistorted during the transmission. To receive the signal successfully, areceiver should compensate for the distortion of the reception signalusing channel information. Generally, to enable the receiver to acquirethe channel information, a transmitter transmits a signal known to boththe transmitter and the receiver and the receiver acquires knowledge ofchannel information based on the distortion of the signal received onthe radio channel. This signal is called a pilot signal or an RS.

In the case of data transmission and reception through multipleantennas, knowledge of channel states between transmission (Tx) antennasand reception (Rx) antennas is required for successful signal reception.Accordingly, an RS should be transmitted through each Tx antenna.

RSs in a mobile communication system may be divided into two typesaccording to their purposes: RS for channel information acquisition andRS for data demodulation. Since its purpose lies in that a UE acquiresdownlink channel information, the former should be transmitted in abroad band and received and measured even by a UE that does not receivedownlink data in a specific subframe. This RS is also used in asituation like handover. The latter is an RS that an eNB transmits alongwith downlink data in specific resources. A UE can estimate a channel byreceiving the RS and accordingly can demodulate data. The RS should betransmitted in a data transmission area.

A legacy 3GPP LTE (e.g., 3GPP LTE release-8) system defines two types ofdownlink RSs for unicast services: a common RS (CRS) and a dedicated RS(DRS). The CRS is used for acquisition of information about a channelstate, measurement of handover, etc. and may be referred to as acell-specific RS. The DRS is used for data demodulation and may bereferred to as a UE-specific RS. In a legacy 3GPP LTE system, the DRS isused for data demodulation only and the CRS can be used for bothpurposes of channel information acquisition and data demodulation.

CRSs, which are cell-specific, are transmitted across a wideband inevery subframe. According to the number of Tx antennas at an eNB, theeNB may transmit CRSs for up to four antenna ports. For instance, an eNBwith two Tx antennas transmits CRSs for antenna port 0 and antenna port1. If the eNB has four Tx antennas, it transmits CRSs for respectivefour Tx antenna ports, antenna port 0 to antenna port 3.

FIG. 6 illustrates a CRS and DRS pattern for an RB (including 14 OFDMsymbols in time by 12 subcarriers in frequency in case of a normal CP)in a system where an eNB has four Tx antennas. In FIG. 6, REs labeledwith ‘R0’, ‘R1’, ‘R2’ and ‘R3’ represent the positions of CRSs forantenna port 0 to antenna port 4, respectively. REs labeled with ‘D’represent the positions of DRSs defined in the LTE system.

The LTE-A system, an evolution of the LTE system, can support up toeight Tx antennas. Therefore, it should also support RSs for up to eightTx antennas. Because downlink RSs are defined only for up to four Txantennas in the LTE system, RSs should be additionally defined for fiveto eight Tx antenna ports, when an eNB has five to eight downlink Txantennas in the LTE-A system. Both RSs for channel measurement and RSsfor data demodulation should be considered for up to eight Tx antennaports.

One of significant considerations for design of the LTE-A system isbackward compatibility. Backward compatibility is a feature thatguarantees a legacy LTE terminal to operate normally even in the LTE-Asystem. If RSs for up to eight Tx antenna ports are added to atime-frequency area in which CRSs defined by the LTE standard aretransmitted across a total frequency band in every subframe, RS overheadbecomes huge. Therefore, new RSs should be designed for up to eightantenna ports in such a manner that RS overhead is reduced.

Largely, new two types of RSs are introduced to the LTE-A system. Onetype is CSI-RS serving the purpose of channel measurement for selectionof a transmission rank, a modulation and coding scheme (MCS), aprecoding matrix index (PMI), etc. The other type is demodulation RS (DMRS) for demodulation of data transmitted through up to eight Txantennas.

Compared to the CRS used for both purposes of measurement such aschannel measurement and measurement for handover and data demodulationin the legacy LTE system, the CSI-RS is designed mainly for channelestimation, although it may also be used for measurement for handover.Since CSI-RSs are transmitted only for the purpose of acquisition ofchannel information, they may not be transmitted in every subframe,unlike CRSs in the legacy LTE system. Accordingly, CSI-RSs may beconfigured so as to be transmitted intermittently (e.g. periodically)along the time axis, for reduction of CSI-RS overhead.

When data is transmitted in a downlink subframe, DM RSs are alsotransmitted dedicatedly to a UE for which the data transmission isscheduled. Thus, DM RSs dedicated to a particular UE may be designedsuch that they are transmitted only in a resource area scheduled for theparticular UE, that is, only in a time-frequency area carrying data forthe particular UE.

FIG. 7 illustrates an exemplary DM RS pattern defined for the LTE-Asystem. In FIG. 7, the positions of REs carrying DM RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. DM RSs maybe transmitted for additionally defined four antenna ports, antenna port7 to antenna port 10 in the LTE-A system. DM RSs for different antennaports may be identified by their different frequency resources(subcarriers) and/or different time resources (OFDM symbols). This meansthat the DM RSs may be multiplexed in frequency division multiplexing(FDM) and/or time division multiplexing (TDM). If DM RSs for differentantenna ports are positioned in the same time-frequency resources, theymay be identified by their different orthogonal codes. That is, these DMRSs may be multiplexed in Code Division Multiplexing (CDM). In theillustrated case of FIG. 7, DM RSs for antenna port 7 and antenna port 8may be located on REs of DM RS CDM group 1 through multiplexing based onorthogonal codes. Similarly, DM RSs for antenna port 9 and antenna port10 may be located on REs of DM RS CDM group 2 through multiplexing basedon orthogonal codes.

FIG. 8 illustrates exemplary CSI-RS patterns defined for the LTE-Asystem. In FIG. 8, the positions of REs carrying CSI-RSs in an RBcarrying downlink data (an RB having 14 OFDM symbols in time by 12subcarriers in frequency in case of a normal CP) are marked. One of theCSI-RS patterns illustrated in FIGS. 8(a) to 8(e) is available for anydownlink subframe. CSI-RSs may be transmitted for eight antenna portssupported by the LTE-A system, antenna port 15 to antenna port 22.CSI-RSs for different antenna ports may be identified by their differentfrequency resources (subcarriers) and/or different time resources (OFDMsymbols). This means that the CSI-RSs may be multiplexed in FDM and/orTDM. CSI-RSs positioned in the same time-frequency resources fordifferent antenna ports may be identified by their different orthogonalcodes. That is, these DM RSs may be multiplexed in CDM. In theillustrated case of FIG. 8(a), CSI-RSs for antenna port 15 and antennaport 16 may be located on REs of CSI-RS CDM group 1 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 17 and antenna port18 may be located on REs of CSI-RS CDM group 2 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 19 and antenna port20 may be located on REs of CSI-RS CDM group 3 through multiplexingbased on orthogonal codes. CSI-RSs for antenna port 21 and antenna port22 may be located on REs of CSI-RS CDM group 4 through multiplexingbased on orthogonal codes. The same principle described with referenceto FIG. 8(a) is applicable to the CSI-RS patterns illustrated in FIGS.8(b) to 8(e).

The RS patterns of FIGS. 6 to 8 are only exemplary, and application ofvarious embodiments of the present invention is not limited to aspecific RS pattern. That is, various embodiments of the presentinvention may equally be applied to even a case where RS patternsdifferent from those of FIGS. 6 to 8 are defined and used.

CSI-RS Configuration

Among the plurality of CSI-RSs and the plurality of IMRs, which areconfigured for the UE, one CSI process may be defined by associating oneCSI-RS resource for signal measurement with one interference measurementresource (IMR) for interference measurement. The UE feeds back CSIinformation derived from different CSI processes to the network (forexample, base station) by using an independent period and subframeoffset.

In other words, each CSI process has independent CSI feedbackconfiguration. Association information on the CSI-RS resource and theIMR resource and CSI feedback configuration may be notified from thebase station to the UE through higher layer signaling such as RRC perCSI process. For example, it is assumed that three CSI processes areconfigured for the UE as illustrated in Table 1.

TABLE 1 Signal Measurement CSI Process Resource (SMR) IMR CSI process 0CSI-RS 0 IMR 0 CSI process 1 CSI-RS 1 IMR 1 CSI process 2 CSI-RS 0 IMR 2

In Table 1, CSI-RS 0 and CSI-RS 1 respectively represent CSI-RS receivedfrom a cell 1 which is a serving cell of the UE and CSI-RS received froma cell 2 which is a neighboring cell which joins cooperation. It isassumed that IMR configured for each CSI process of Table 1 isconfigured as illustrated in Table 2.

TABLE 2 IMR eNB 1 eNB 2 IMR 0 Muting Data transmission IMR 1 Datatransmission Muting IMR 2 Muting Muting

In IMR 0, the cell 1 performs muting, the cell 2 performs datatransmission, and the UE is configured to measure interference from theother cells except the cell 1. Likewise, in IMR 1, the cell 2 performsmuting, the cell performs data transmission, and the UE is configured tomeasure interference from the other cells the cell 2. Also, in IMR 2,both the cell 1 and the cell 2 perform muting, and the UE is configuredto measure interference from the other cells except the cell 1 and thecell 2.

Accordingly, as illustrated in Table 1 and Table 2, CSI information ofthe CSI process 0 represents optimized RI, PMI and CQI information ifdata are received from the cell 1. CSI information of the CSI process 1represents optimized RI, PMI and CQI information if data are receivedfrom the cell 2. CSI information of the CSI process 2 representsoptimized RI, PMI and CQI information if data are received from the cell1 and if there is no interference from the cell 2.

It is preferable that a plurality of CSI processes configured for one UEshare dependent values. For example, in case of joint transmission (JP)of the cell 1 and the cell 2, if a CSI process 1 which regards a channelof the cell 1 as a signal part and a CSI process 2 which regards achannel of the cell 2 as a signal part are configured for one UE, theCSI process 1 and the CSI process 2 need to have the same rank andsubband indexes in order to easily perform JT scheduling.

A period or pattern for transmission of the CSI-RS may be configured bythe base station. In order to measure the CSI-RS, the UE should knowCSI-RS configuration for each CSI-RS antenna port of a cell to which theUE belongs. The CSI-RS configuration may include a downlink subframeindex to which the CSI-RS is transmitted, time-frequency location (forexample, CSI-RS pattern as shown in FIGS. 8(a) to 8(e)) of a CSI-RSresource element (RE) within a transmission subframe, and a CSI-RSsequence (sequence used for CSI-RS and generated pseudo-randomly inaccordance with a predetermined rule on the basis of a slot number, cellID, a CP length, etc.). That is, a plurality of CSI-RS configurationsmay be used by a given base station, and the base station may notify UEswithin a cell of a CSI-RS configuration, which will be used for the UEs,among a plurality of CSI-RS configurations.

Also, since the CSI-RS for each antenna port is needed to be identifiedfrom another one, resources to which the CSI-RS for each antenna port istransmitted should be orthogonal to one another. As described withreference to FIG. 8, the CSI-RSs for each antenna port may bemultiplexed in accordance with an FDM mode, a TDM mode and/or a CDM modeby using orthogonal frequency resources, orthogonal time resourcesand/or orthogonal code resources.

When the base station reports information (CSI-RS configuration) onCSI-RS to UEs within a cell, the base station should first notify theUEs of information on time-frequency into which the CSI-RS for eachantenna port is mapped. In more detail, the information on time mayinclude subframe numbers to which the CSI-RS is transmitted, atransmission period of the CSI-RS, subframe offset for transmission ofthe CSI-RS, and OFDM symbol number to which a CSI-RS resource element(RE) of a specific antenna is transmitted. The information on frequencymay include a frequency spacing to which the CSI-RS resource element(RE) of a specific antenna is transmitted, offset or shift value of REon a frequency axis, etc.

FIG. 9 is a diagram illustrating an example of periodically transmittinga CSI-RS. The CSI-RS may be transmitted periodically with a period (forexample, a period of 5 subframes, a period of 10 subframes, a period of20 subframes, a period of 40 subframes, or a period of 80 subframes) ofan integer multiple of one subframe.

In FIG. 9, one radio frame includes 10 subframes (subframe numbers 0 to9). For example, in FIG. 9, a transmission period of the CSI-RS of thebase station is 10 ms (that is, 10 subframes), and CSI-RS transmissionoffset is 3. Each offset value may be varied for each base station,whereby CSI-RSs of various cells may uniformly be distributed on thetime. If the CSI-RS is transmitted with a period of 10 ms, the offsetvalue may have one of 0 to 9. Similarly, if the CSI-RS is transmittedwith a period of 5 ms, the offset value may have one of 0 to 4, if theCSI-RS is transmitted with a period of 20 ms, the offset value may haveone of 0 to 19, if the CSI-RS is transmitted with a period of 40 ms, theoffset value may have one of 0 to 39, and if the CSI-RS is transmittedwith a period of 80 ms, the offset value may have one of 0 to 79. Thisoffset value represents a value of a subframe at which the base stationstarts CSI-RS transmission with a predetermined period. If the basestation notifies the UE of a transmission period and offset value of theCSI-RS, the UE may receive the CSI-RS of the base station at thelocation of the corresponding subframe by using the notified value. TheUE measures a channel through the received CSI-RS, and as a result, mayreport information such as CQI, PMI and/or RI (Rank Indicator) to thebase station. Herein, CQI, PMI and RI may collectively be referred to asCQI (or CSI) except that CQI, PMI and RI are described separately. Also,the transmission period and offset of the CSI-RS may separately bedesignated per CSI-RS configuration.

FIG. 10 is a diagram illustrating an example of non-periodicallytransmitting a CSI-RS. In FIG. 10, one radio frame includes 10 subframes(subframe numbers 0 to 9). A subframe to which the CSI-RS is transmittedmay be represented by a specific pattern as shown in FIG. 10. Forexample, a CSI-RS transmission pattern may be configured in a unit of 10subframes, and CSI-RS transmission at each subframe may be designated bya 1-bit indicator. A CSI-RS pattern transmitted at subframe indexes 3and 4 within 10 subframes (subframe indexes 0 to 9) is shown in theexample of FIG. 10. The indicator may be provided to the UE throughhigher layer signaling.

Various configurations for CSI-RS transmission may be configured asdescribed above. In order that the UE performs channel measurement bynormally receiving the CSI-RS, the base station needs to notify the UEof CSI-RS configuration. Embodiments of the present invention related tonotification of CSI-RS configuration to the UE will be describedhereinafter.

Notification Scheme of CSI-RS Configuration

Generally, as schemes for enabling a base station to notify a UE ofCSI-RS configuration, two schemes may be considered as follows.

The first scheme is that the base station broadcasts information onCSI-RS configuration to UEs by using dynamic broadcast channel (DBCH)signaling.

In the legacy LTE system, when notifying the UEs of a message related tosystem information, the base station may generally transmit thecorresponding information through a BCH (Broadcast Channel). If the basestation cannot transmit the message related to the system informationthrough the BCH only due to too much message related to the systeminformation, the base station may transmit the system information likegeneral downlink data, wherein PDCCH CRC of corresponding data is maskedusing system information identifier (SI-RNTI) not a specific UEidentifier (for example, C-RNTI) to transmit the system information. Inthis case, the actual system information is transmitted on a PDSCHregion like general unicast data. Therefore, all the UEs in a cell maydecode a PDCCH by using SI-RNTI and then acquire system information bydecoding a PDSCH indicated by the corresponding PDCCH. The broadcastingscheme as above may be referred to as DBCH (Dynamic BCH) to beidentified from PBCH (Physical BCH) which is a general broadcastingscheme.

Meanwhile, the system information broadcasted in the legacy LTE systemmay be categorized into two types. One type is a master informationblock (MIB) transmitted through the PBCH, and the other one type is asystem information block (SIB) transmitted by being multiplexed withgeneral unicast data on the PDSCH region. Since information transmittedas SIB type 1 to SIB type 8 (SIB1 to SIB8) is defined in the legacy LTEsystem, a new SIB type may be defined for information on CSI-RSconfiguration, which is new system information which is not defined inthe legacy SIB types. For example, SIB9 to SIB10 may be defined, wherebythe base station may notify the UEs within a cell of information onCSI-RS configuration in accordance with the DBCH scheme.

The second scheme is that the base station broadcasts information onCSI-RS configuration to each UE by using RRC (Radio Resource Control)signaling. That is, information on CSI-RS configuration may be providedto each of the UEs within a cell by using dedicated RRC signaling. Forexample, while the UE is establishing connection with the base stationthrough initial access or handover, the base station may broadcastCSI-RS configuration to the corresponding UE through RRC signaling.Alternatively, when transmitting RRC signaling message, which requeststhe UE of channel state feedback based on CSI-RS measurement, to the UE,the base station may notify the corresponding UE of CSI-RS configurationthrough the corresponding RRC signaling message.

Indication of CSI-RS Configuration

A plurality of CSI-RS configurations may be used by a given basestation, and the base station may transmit a CSI-RS based on each CSI-RSconfiguration to the UE on a predetermined subframe. In this case, thebase station may notify the UE of the plurality of CSI-RSconfigurations, and may notify the UE of CSI-RS which will be used forchannel state measurement for CQI (Channel Quality Information) or CSI(Channel State Information) feedback.

The embodiment related to indication of CSI-RS configuration, which willbe used by the UE, and of a CSI-RS, which will be used for channelmeasurement, from the base station will be described hereinafter.

FIG. 11 is a diagram illustrating an example of two CSI-RSconfigurations which are used. FIG. 11 illustrates that one radio frameincludes 10 subframes (subframe numbers 0 to 9). In FIG. 11, a firstCSI-RS configuration, that is, a CSI-RS1 has a CSI-RS transmissionperiod of 10 ms and a CSI-RS transmission offset of 3. In FIG. 11, asecond CSI-RS configuration, that is, a CSI-RS2 has a CSI-RStransmission period of 10 ms and a CSI-RS transmission offset of 4. Thebase station may notify the UE of information on two CSI-RSconfigurations, and may notify the UE which one of the two CSI-RSconfigurations will be used for CQI (or CSI) feedback.

If the UE receives a request of CQI feedback for a specific CSI-RSconfiguration from the base station, the UE may perform channel statemeasurement by using a CSI-RS only which belongs to the correspondingCSI-RS configuration. In more detail, the channel state is determined byCSI-RS received quality and a function of the amount ofnoise/interference and a correlation coefficient, wherein CSI-RSreceived quality is measured by using the CSI-RS only which belongs tothe corresponding CSI-RS configuration, and the amount ofnoise/interference and the correlation coefficient (for example,interference covariance matrix indicating a direction of interference)may be measured at a corresponding CSI-RS transmission subframe ordesignated subframes. For example, in the embodiment of FIG. 11, if theUE receives a request of feedback on the first CSI-RS configuration(CSI-RS1) from the base station, the UE may perform received qualitymeasurement by using the CSI-RS transmitted at the fourth subframe(subframe index 3) of one radio frame, and may be designated toseparately use an odd numbered subframe to measure the amount ofnoise/interference and the correlation coefficient. Alternatively, theUE may be designated to perform CSI-RS received quality measurement andmeasure the amount of noise/interference and the correlation coefficientby being restricted to a specific single subframe (for example, subframeindex 3).

For example, received signal quality measured using the CSI-RS may beexpressed briefly by S/(I+N) (wherein S is strength of a receivedsignal, I is the amount of interference, and N is the amount of noise)as a signal-to-interference plus noise ratio (SINR). S may be measuredthrough the CSI-RS at a subframe that includes the CSI-RS transmitted tothe corresponding UE. Since I and N are varied depending on the amountof interference from a neighboring cell, a direction of a signal fromthe neighboring cell, etc., I and N may be measured through a CRStransmitted at a subframe that measures S or a subframe which isdesignated separately.

In this case, measurement of the amount of noise/interference and thecorrelation coefficient may be performed at the resource element (RE) towhich the CRS or CSI-RS within the corresponding subframe istransmitted, or may be performed through a null RE configured tofacilitate measurement of noise/interference. In order to measurenoise/interference at the CRS or CSI-RS RE, the UE first recovers theCRS or CSI-RS and then removes the recovered result from the receivedsignal to allow noise and interference signals only to remain, whereby astatistic value of noise/interference may be obtained. The null RE meansan empty RE (that is, RE of which transmission power is 0 (zero)) wherethe corresponding base station does not transmit any signal, andfacilitates signal measurement from another base station except thecorresponding base station. Although all of the CRS RE, the CSI-RS REand the null RE may be used to measure the amount of noise/interferenceand the correlation coefficient, the base station may designate REs,which will be used to measure noise/interference, for the UE. This isbecause that it is required to appropriately designate RE which will bemeasured by the corresponding UE depending on whether a signal of aneighboring cell, which is transmitted to the location of the RE wherethe UE performs measurement, is a data signal or a control signal. Sincethe signal of the neighboring cell, which is transmitted to the locationof the corresponding RE, is varied depending on inter-cellsynchronization, CRS configuration, and CSI-RS configuration, the basestation may identify the corresponding signal and then designate the RE,which will perform measurement, for the UE. That is, the base stationmay designate all or some of the CRS RE, the CSI-RS RE and the null REfor the UE to measure noise/interference by using the designated RE(s).

For example, the base station may use the plurality of CSI-RSconfigurations, and may notify the UE of a CSI-RS configuration and alocation of a null RE, which will be used for CQI feedback, whilenotifying the UE of one or more CSI-RS configurations. The CSI-RSconfiguration which will be used for CQI feedback by the UE may beexpressed as CSI-RS configuration transmitted at a non-zero transmissionpower in view of an aspect identified from the null RE transmitted at atransmission power of 0. For example, the base station may notify the UEof one CSI-RS configuration through which the UE will perform channelmeasurement, and the UE may assume that the CSI-RS is transmitted at anon-zero transmission power in the one CSI-RS configuration.Additionally, the base station may notify the UE of the CSI-RSconfiguration transmitted at a transmission power of 0, and the UE mayassume that the location of the RE of the corresponding CSI-RScorresponds to the transmission power of 0. In other words, the basestation may notify the UE of the location of the corresponding null REif the CSI-RS configuration of the transmission power of 0 exists whilenotifying the UE of one CSI-RS configuration of the non-zerotransmission power.

As a modified example of the aforementioned indication of the CSI-RSconfiguration, the base station may notify the UE of the plurality ofCSI-RS configurations, and may notify the UE of all or some of theCSI-RS configurations, which will be used for CQI feedback. Therefore,the UE which is requested CQI feedback for the plurality of CSI-RSconfigurations may measure CQI by using the CSI-RS corresponding to eachCSI-RS configuration and transmit the measured CQI information to thebase station.

Otherwise, the base station may designate uplink resources required forCQI transmission previously for each CSI-RS configuration, whereby theUE may transmit CQI for each of the plurality of CSI-RS configurationsto the base station. Information on designation of the uplink resourcesmay previously be provided to the UE through RRC signaling.

Otherwise, the base station may dynamically trigger CQI for each of theplurality of CSI-RS configurations to allow the UE to transmit the CQIto the base station. Dynamic triggering of CQI transmission may beperformed through the PDCCH. A corresponding CSI-RS configuration forwhich CQI measurement will be performed will be notified to the UEthrough the PDCCH. The UE that has received the PDCCH may feed theresult of CQI measurement for the CSI-RS configuration designated in thecorresponding PDCCH back to the base station.

A transmission timing of the CSI-RS corresponding to each of theplurality of CSI-RS configurations may be designated such that theCSI-RS is transmitted at different subframes or the same subframe. IfCSI-RS transmission based on different CSI-RS configurations isdesignated at the same subframe, it is required to identify the CSI-RSsfrom one another. In order to identify the CSI-RSs based on differentCSI-RS configurations from one another, one or more of time resources,frequency resources and code resources of CSI-RS transmission may beapplied differently. For example, the location of the RE where theCSI-RS is transmitted at the corresponding subframe may be designateddifferently (for example, the CSI-RS based on one CSI-RS configurationis transmitted at the location of the RE in FIG. 8(a), and the CSI-RSbased on the other CSI-RS configuration is transmitted at the locationof the RE in FIG. 8(b) at the same subframe) for each CSI-RSconfiguration (identification based on time and frequency resources).Alternatively, if CSI-RSs based on different CSI-RS configurations aretransmitted at the same location of the RE, CSI-RS scrambling codes maybe used differently for different CSI-RS configurations, whereby theCSI-RSs may be identified from one another (identification based on coderesources).

Quasi Co-Located (QC)

The UE may receive data from a plurality of transmission points (TPs),for example, TP1 and TP2. Therefore, the UE may transmit channel stateinformation on the plurality of TPs. In this case, RSs may betransmitted from the plurality of TPs to the UE. At this time, ifproperties for channel estimation from different RS ports of differentTPs are shared between the TPs, load and complexity of receivingprocessing of the UE may be lowered. Moreover, if properties for channelestimation from different RS ports of the same TP are shared between theRS ports, load and complexity of receiving processing of the UE may belowered. In this respect, the LTE-A system suggests a method for sharingproperties for channel estimation between RS ports.

For channel estimation between RS ports, the LTE-A system has introducedthe concept of “quasi co-located (QCL)”. For example, if a large-scaleproperty of a radio channel to which a symbol is transmitted through oneantenna port is inferred from a radio channel to which a symbol istransmitted through another antenna port, the two antenna ports may bequasi co-located. In this case, the large-scale property includes one ormore of delay spread, Doppler spread, Doppler shift, average gain, andaverage delay. Hereinafter, the quasi co-located will simply be referredto as QCL.

In other words, if the two antenna ports are subjected to QCL, it meansthat the large-scale property of the radio channel from one antenna portis the same as that of a radio channel from the other one antenna port.If the antenna ports to which two different types of RSs are transmittedare subjected to QCL, the large-scale property of the radio channel fromone antenna port may be replaced with that of a radio channel from theother one antenna port.

In accordance with the concept of QCL, the UE cannot assume the samelarge-scale property between radio channels from non-QCL antenna ports.That is, in this case, the UE should perform independent processing foreach non-QCL antenna port configured for timing acquisition tracking,frequency offset estimation and compensation, delay estimation andDoppler estimation.

It is advantageous in that the UE may perform the following operationsfor antenna ports that may assume QCL. First of all, the UE may use theresult of delay spread, Doppler spectrum, and Doppler spread estimationfor a radio channel from one antenna port during channel estimation fora radio channel from another antenna port. Next, regarding frequencyshift and received timing, the UE may perform time and frequencysynchronization for one antenna port and then apply the samesynchronization to demodulation of another antenna port. Next, regardingaverage received power, the UE may average RSRP (Reference SignalReceived Power) measurement for two or more antenna ports.

If the UE receives a DMRS-based downlink-associated DCI format through acontrol channel (PDCCH or ePDCCH), the UE performs channel estimationfor the corresponding PDSCH through DMRS sequence and then performs datademodulation. For example, if a configuration of a DMRS port received bythe UE from a downlink scheduling grant may be subjected to QCLassumption with a CRS port, the UE may apply an estimation value of alarge-scale property of a radio channel estimated from the CRS portduring channel estimation through the corresponding DMRS port as it is.This is because that the estimation value for the large-scale propertymay be acquired from the CRS more stably because the CRS is a referencesignal broadcasted at a relatively high density over a full band persubframe. On the other hand, the DMRS is transmitted UE-specifically fora specific scheduled RB, and a precoding matrix used for transmission bythe base station may be varied in a unit of PRG, whereby a valid channelreceived by the UE may be varied in a unit of PRG. Therefore,performance degradation may be generated when the DMRS is used forestimation of the large-scale property of the radio channel over a broadband. Since the CSI-RS has a relatively long transmission period and lowdensity, performance degradation may also be generated when the CSI-RSis used for estimation for the large-scale property of the radiochannel.

That is, QCL assumption between the antenna ports may be used forreception of various downlink reference signals, channel estimation,channel state report, etc.

Method for Cancelling Interference

FIG. 12 is a diagram illustrating a general interference environment ofa downlink system.

For convenience of description, a cell controlled by a TP A will bereferred to as a cell A, and a user equipment which performscommunication with the TP A will be referred to as UE a. Likewise, acell B and a UE b exist for a neighboring TP B. Since the cell A and thecell B use the same radio resource, the UE b which is located at thecell edge is subjected to interference from the cell A. Hereinafter, thecell A will be referred to as an interference cell, the TP A will bereferred to as an interfering TP, the cell B will be referred to as aserving cell, the TP B will be referred to as a serving TP, and the UE bwill be referred to as an NAICS UE. The NAICS UE may increase a datareceived rate by cancelling an interference signal from the interferencecell.

The NAICS UE should know various kinds of information (IP, interferenceparameter) for the interference signal to effectively cancelinterference. For example, in an NAICS environment which is independentfrom a transmission mode (TM), information of CFI, MBSFN configuration,RI, CRS AP, Cell ID, Modulation order, MCS, RNTI, and TM is required. Incase of an NAICS environment of CRS TM, information of PMI, Data to RSEPRE, PA, PB, System bandwidth, and PDSCH allocation is required. Also,in case of an NAICS environment of DM-RS TM, information of PDSCHbandwidth for DM-RS, Data to RS EPRE, PB, DMRS APs, nSCID, CSI-RSpresence and their pattern, and Virtual cell ID is required.

The NAICS UE cancels the interference signal by receiving theaforementioned information on the interference signal through theserving TP or the interfering TP or discovering the information throughblind detection (BD). However, signaling overhead and complexity may beincreased to receive all interference parameters (IP) which arerequired. Also, if BD is performed for some of the IP, an incorrectvalue may be detected, whereby the interference signal may not becanceled normally.

As a solution, values of some of the IP may be restricted previouslythrough network coordination. In more detail, a method for previouslyrestricting values of IP through network coordination will be suggestedas follows.

First Embodiment

First of all, according to the first embodiment, the number ofinterference layers can be restricted.

In more detail, the number of interference layers may be restricted tosatisfy two conditions as follows. An interfering base stationdetermines the number of interference layers, which satisfies one of thefollowing two conditions or both of them, by receiving the number ofdesired layers and the number of receiving antennas of an NAICS UE froma serving base station.

The first condition is that the number of interference layers reachesthe number of desired layers or less.

The number of data layers of the interfering TP should be set to reachthe number of layers of desired data or less. Generally, since amagnetic channel is stronger than the interfering channel, it isrequired to set the number of layers of desired data to a maximum valueof a data layer of the interfering TP. Also, since the number of layersof a magnetic signal is leveled up by reflecting the effect ofinterference cancellation but the interference signal is demodulated ordecoded in a state that the magnetic signal exists as interference, itis preferable that the number of interference layers is set to besmaller than the number of layers of the magnetic signal.

The serving base station determines the number of layers of a desiredsignal transmitted to the NAICS UE and then forwards the determinedresultant value to the interfering base station, whereby the interferingbase station should set the number of layers for its DL data to thedetermined resultant value or less. The NAICS UE performs BD for aninterference signal rank corresponding to the determined resultant valueor less with reference to a rank of desired data from DCI.

The second condition is that the number of interference layers reaches avalue, which is obtained by subtracting the number of desired layersfrom the number of receiving antennas, or less.

A maximum rank for transmission and reception in an M by N SISO channelis determined by min (M, N) corresponding to degree of freedom of achannel. Based on this theoretical background, the base stationgenerally sets a rank to a minimum value or less of the number oftransmitting antennas and the number of receiving antennas of the UE andthen transmits data to the UE. However, since the NAICS UE shouldperform demodulation or decoding for the interference signal as well asits data, it may be difficult to cancel interference in accordance witha rank of the interference signal even through the rank of the desiredsignal is determined normally. Therefore, the NAICS UE may normallyperform interference cancellation when a sum of a rank for data of theNAICS UE and a rank of the interference signal is set to a receivingspatial domain of the NAICS UE, that is, the number of receivingantennas or less.

The serving base station determines the number of layers of the desiredsignal transmitted to the NAICS UE and then forwards the determinedvalue and the number of receiving antennas of the NAICS UE or “thenumber of receiving antennas of the NAICS UE—the number of layers of thedesired signal” to the interfering base station. Therefore, theinterfering base station may set the number of layers for its DL data to“the number of receiving antennas of the NAICS UE—the number of layersof the desired signal” or less. The NAICS UE performs BD for aninterference signal rank corresponding to a value of ‘the number of itsreceiving antennas—the number of layers of the desired signal’ or lesswith reference to a rank of desired data from DCI.

Although the above two conditions express the maximum value of thenumber of interference layers, the number of interference layers may bedetermined directly based on the number of receiving antennas and thenumber of desired layers. That is, “the number of receiving antennas—thenumber of desired layers” may be set to the number of interferencelayers, or the number of desired layers may be set to the number ofinterference layers. In this case, the UE can calculate the number ofinterference layers without BD.

The above two conditions may be normalized to set the number ofinterference layers to a function of the number of receiving antennasand the number of desired layers. That is, the number of interferencelayers is scheduled to be smaller than a function value based on thenumber of receiving antennas and the number of desired layers betweenthe base station and the UE, and the UE can reduce BD candidates of thenumber of interference layers in accordance with the scheduled value. Ifthe number of interference layers not the maximum value of the number ofinterference layers is determined directly by the above two factors,that is, if the number of interference layers is determined by afunction value based on the number of receiving antennas and the numberof desired layers, the UE can calculate the number of interferencelayers without BD.

More similarly, the number of interference layers may be fixed to 1 orlimited to N or less. For example, if a specific subframe for performingNAICS is determined previously, the interfering base station may fix thenumber of interference layers to 1 or limit the number of interferencelayers to N or less at the corresponding subframe. The NAICS UE performsICS by assuming the number of interference layers to 1 at thecorresponding subframe or assuming the number of interference layers toN or less.

Second Embodiment

According to the second embodiment, a modulation order or a modulationand coding scheme of interference data can be restricted.

In more detail, the modulation order or the modulation and coding schemeof the interference signal can be restricted to satisfy the followingconditions. The interfering base station determines the modulationorder/MCS of the interference signal, which satisfies the followingcondition, by receiving a modulation order/MCS of a desired signal fromthe serving base station.

The restriction condition is that the modulation order/MCS of theinterference signal is restricted to the modulation order/MCS or less ofthe desired signal.

A modulation order/MCS of the interfering TP should be set to amodulation order/MCS of the desired data. Generally, since the magneticchannel is stronger than the interfering channel, it is required to setthe modulation order/MCS of the desired data to set a maximum value ofthe modulation order/MCS of the interfering TP. Also, although themodulation order/MCS of the magnetic signal is leveled up by reflectinginterference cancellation effect, the interference signal is demodulatedor decoded in a state that the magnetic signal exists as interference.Therefore, it is preferable that the modulation order/MCS of theinterference signal is set to be smaller than the modulation order/MCSof the magnetic signal.

The serving base station determines the modulation order/MCS of thedesired signal transmitted to the NAICS UE and then forwards thedetermined result to the interfering base station. The interfering basestation sets the modulation order/MCS for its DL data to the modulationorder/MCS or less of the desired signal. The NAICS UE performs BD forthe modulation order/MCS value or less of the desired signal withreference to the modulation order/MCS of the desired data from the DCIwhen performing BD for the modulation order/MCS of the interferencesignal.

Although the above condition restricts the maximum value of themodulation order/MCS of the interference signal, the modulationorder/MCS value of the interference signal may be determined based onthe modulation order/MCS of the desired signal. That is, networkcoordination may be performed such that the modulation order/MCS of thedesired signal may be set to the modulation order/MCS of theinterference signal. At this time, the UE can calculate the modulationorder/MCS of the interference signal without BD.

The above condition may be normalized such that the modulation order/MCSof the interference signal may be set to the function of the modulationorder/MCS of the desired signal. That is, it is scheduled between thebase station and the UE that the modulation order/MCS of theinterference signal is smaller than the function based on the modulationorder/MCS of the desired signal, and the UE can reduce BD candidates ofthe modulation order/MCS of the interference signal in accordance withthe scheduled value. Also, the modulation order/MCS of the interferencesignal is determined by a function value based on the demodulationorder/MCS of the desired signal, the UE can calculate the demodulationorder/MCS of the interference signal without BD.

More similarly, the modulation order/MCS of the interference signal maybe limited to N or less. For example, if a specific subframe forperforming NAICS is determined previously, the interfering base stationrestricts modulation to QPSK at the corresponding subframe. The NAICS UEperforms ICS by assuming modulation of the interference signal at thecorresponding subframe as QPSK.

Third Embodiment

According to the third embodiment, precoding of interference data can berestricted.

In order that the NAICS UE cancels an interference signal by normallyperforming demodulation and decoding, a received power of theinterference signal should be high. Therefore, if the UE feeds backoptimized PMI for optimizing channel gain by measuring a channel with aninterference cell, the interfering base station determines precodingwith reference to the feedback value. The PMI feedback may directly beforwarded from the UE to the interfering base station, or may beforwarded to the interfering base station through a backhaul link afterbeing forwarded from the UE to the serving base station.

The UE can determine the interference cell PMI based on the number ofinterference layers according to the first embodiment. For example, theUE determines the interference cell PMI by assuming a value obtained bysubtracting the number of layers of desired data from the number ofreceiving antennas as the number of interference layers. Alternatively,the UE can calculate the interference cell PMI by assuming a rank as 1.The interference cell that has received the PMI performs precoding byusing the PMI. The UE assumes that the interference cell has performedprecoding to the interference cell PMI prior to a subframe n-N whenperforming NAICS at a subframe n. Alternatively, signaling indicatingwhether the interference cell has performed precoding for aninterference cell PMI most recently reported prior to the subframe n maybe added when the UE performs NAICS at the subframe n. This signalingmay be added within the DCI as 1 bit. If the corresponding value is ‘1’,the UE assumes that the interference cell has performed precoding to theinterference cell PMI most recently reported prior to the subframe n.

Also, the UE may report a plurality of PMIs without reporting a specificPMI only to the interfering base station. As the plurality of PMIs arereported to the interfering base station, the interfering base stationcan select the optimized PMI of more PMIs. For example, the UE reportsPMI causing the strongest interference and PMI causing the second stronginterference to the interfering base station. Alternatively, the UEreports interference PMI assuming a rank 1 and interference PMI assuminga rank 2 to the interfering base station. The interfering base stationthat has received the interference PMIs can perform scheduling for oneof the two PMIs. The UE discovers an interference cell precoder byperforming BD for the two interference cell PMIs reported prior to thesubframe n-N when performing NAICS at the subframe n.

Although the UE has reported the interference PMI causing the strongestinterference as above, the UE may report PMI causing weakestinterference. In this case, the interfering base station performsprecoding by using either this PMI causing the weakest interference orone of a set of PMIs having low correlation with the PMI causing theweakest interference. If the interfering base station performs precodingby using one of a set of PMIs having low correlation with the PMIcausing the weakest interference, the UE discovers an interference cellprecoder by performing BD for a PMI set having low correlation withrespect to the interference cell PMI reported prior to the subframe n-Nwhen performing NAICS at the subframe n. PMI set information having lowcorrelation with respect to a specific PMI may previously be sharedbetween the base station and the UE. Alternatively, when the UE performsNAICS at the subframe n, signaling indicating whether the interferencecell has performed precoding by using the interference PMI set mostrecently reported prior to the subframe n may be added. This signalingmay be added within the DCI as 1 bit. If the corresponding value is ‘1’,the UE assumes that the interference cell has performed precoding byusing the interference cell PMI most recently reported prior to thesubframe n, and may reduce a target for BD.

The interference PMI may be fixed regardless of the interfering channelmore simply than a method for determining interference PM in accordancewith an interference channel between the NAICS UE and the interferingbase station. For example, if a subframe for performing NAICS isscheduled previously between the base station and the UE, PMI, which maybe used by the interference cell, may be fixed at the correspondingsubframe. That is, PMI that may be used for RB i by the interferencecell at the subframe for performing NAICS is limited to PMI set i. TheUE discovers a precoder, which is used by the interference cell, byperforming BD for the PMI set i when performing NAICS at RB i. Forexample, the interference cell may be restricted that the interferencecell uses an even numbered PMI only at an even numbered RB and uses anodd numbered PMI only at an odd numbered RB.

Fourth Embodiment

According to the fourth embodiment, a PDSCH starting symbol can berestricted.

The NAICS UE may discover interference PDSCH starting symbol informationby performing BD for a PCIFCH of the interference cell. However, theNAICS UE may not discover interference PDSCH starting symbol informationas the case may be. For example, when the interference cell performs CAby using two CCs and cross carrier scheduling is performed, a PDSHstarting symbol of an Scell cannot be derived from the PCIFCH. This isbecause that the interference cell UE served from the Scell isseparately configured for the PDSCH starting symbol through RRC, and thePDSCH starting symbol is determined in accordance with this value. Foranother example, if the interference cell transmits EPDCCH, the PDSCHstarting symbol is determined at the same symbol as an EPDCCH startingsymbol of the interference cell.

Considering this case, the base station may perform RRC indication tothe NAICS UE, which indicates whether an interference PDSCH startingsymbol can be derived by the interference PCFICH. That is, if a bitvalue through RRC is 1, the NAICS UE determines that the interferencePDSCH starting symbol can be derived by the interference PCFICH, andperforms interference PCFICH BD. If the bit value is 0, the NAICS UEdetermines that the interference PDSCH starting symbol cannot be derivedby the interference PCFICH and does not perform BD for the interferencePCFICH. In case of the latter case, the NAICS UE may discover the PDSCHstarting symbol through BD or assume a maximum number of interferencePDSCH symbols and assume next symbol as the PDSCH starting symbol.

Whether the interference PDSCH starting symbol can be derived by theinterference PCFICH can be defined in the NAICS UE as one state togetherwith a PDSCH IC starting symbol as illustrated in Table 3 below.

TABLE 3 bit fields descriptions 001 follow PCFICH 010 n = 2 011 n = 3100 n = 4 101 n = 5 000, 110, 111 reserved

In Table 3, a bit field 001 illustrates that BD is performed for thePCFICH.

The PDSCH IC starting symbol may be defined that the interference celldoes not assure transmission of the PDSCH for next symbol including asymbol n instead of indicating an actual PDSCH starting position used bythe interference cell. That is, PDSCH transmission may be performed ornot prior to the symbol n. Therefore, TP can dynamically change theactual PDSCH starting position within the range of symbols 1 to n. As aresult, the UE may try to cancel interference from the PDSCH IC startingsymbol without performing BD for the PCFICH.

FIG. 13 is a diagram illustrating an example of PDSCH interferencecancellation based on a PDSCH IC starting symbol according to thepresent invention.

Referring to FIG. 13, if a symbol is configured for the NAICS UE as thePDSCH IC starting symbol, the UE may try to cancel interference in aPDSCH IC region of the interference PDSCH regardless of the actual PDSCHstarting position.

The symbol n transmitted in the state of Table 3 may be configured asthe PDSCH starting symbol, which is used by the actual interfering basestation, not the PDSCH IC starting symbol.

Alternatively, when the UE transmits the PDSCH IC starting symbol onlythrough RRC signaling as illustrated in Table 4 below and derives theinterference PDSCH starting symbol through the interference PCFICH, theUE can be scheduled not to transmit this information. That is, if the UEdoes not receive information of Table 4, the UE can derive theinterference PDSCH starting symbol by performing BD for the interferencePCFICH, and the network can assure an environment that the UE candiscover interference PDSCH starting symbol information by performing BDfor PCFICH of the interference cell.

TABLE 4 bit fields descriptions 00 n = 2 01 n = 3 10 n = 4 11 n = 5

Fifth Embodiment

According to the fifth embodiment, QCL can be restricted.

In order to increase channel estimation performance based oninterference DM-RS sequence, the base station provides QCL assumption.QCL assumption is to improve channel estimation performance of DM-RSsequence by using channel property value of another RS having channelproperty the same as or similar to that of the corresponding sequencewhile having RS density higher than that of DM-RS in estimating achannel of a corresponding interference DM-RS sequence.

More exactly, QCL (Quasi co-location) for each antenna port is definedin the 3GPP standard. Two behaviors are defined for QCL. First of all, abehavior A is that a CRS, a DM-RS and a CSI-RS are transmitted from theserving cell and all the antenna ports have the same channel property.Next, a behavior B is defined that the DM-RS has the same channelproperty as that of a specific CSI-RS in demodulation of a PDSCH. In thebehavior B, QCL with a specific CRS as well as QCL with the DM-RS andthe CSI-RS may additionally be signaled to the UE.

Therefore, QCL assumption may be transmitted to each sequence in aninterference DM-RS sequence candidate group. Simply, the DM-RS sequenceand specific non-zero power CSI-RS index may be mapped.

Since the method according to the present invention is not limited to aCoMP structure defined in the 3GPP Rel-11, CSI-RS index is not limitedto the CSI-RS configured for CSI feedback of the UE in the CoMPstructure. Since the UE does not need to necessarily perform CSIfeedback for the CSI-RS index which is signaled, a problem may occur inthat the CSI-RS to be measured by the UE is increased unnecessarily toobtain channel property as information which is helpful for estimationof a specific DM-RS sequence. Therefore, QCL information intended forchannel estimation for the interference DM-RS sequence will beunderstood more preferably with reference to information from a CRS of aspecific cell in addition to the CSI-RS. That is, for QCL assumption,the CSI-RS index or PCID of a specific cell may be notified to obtainchannel property from the CRS of the corresponding cell.

That is, although the CoMP UE performs DMRS channel estimation byassuming QCL B to receive TM10 service, the NAICS UE may be restrictedto perform interference DMRS channel estimation by using QCL A only forTM10 interference signal.

To this end, the base station may explicitly transmit RRC indication,which indicates whether QCL A can be used for TM10 interference, to theNAICS UE.

Alternatively, the following method may be considered without explicitindication. The base station may select an interference CRS and aninterference DMRS, which may be connected to QCL A, and may notifyinformation of the corresponding DMRS as network assistance information.In this case, since the base station assures QCL A of the interferenceDMRS, the UE discovers delay spread, Doppler spread, Doppler shift andaverage delay information from the CRS associated with DMRS VCID(virtual cell ID) by always assuming QCL A when performing NAICS for theTM10 interference signal, and then uses the discovered information forDMRS channel estimation.

For example, the base station sends interference CRS assistanceinformation and DMRS assistance information as shown in FIGS. 14 and 15,and notifies DMRS and CRS subjected to QCL A through qcl-CRS-Info. TheUE performs BD for interference VCID and then discovers the interferenceCRS subjected to QCL through qcl-CRS-Info of the detected VCID. At thistime, the UE assumes that the DMRS of the corresponding VCID is alwaysconnected to the corresponding CRS through QCL A.

As described above, the UE always assumes QCL A for a DMRS basedinterference PDSCH, that is, an interference PDSCH transmitted usingsome of ports 7 to 14, and assumes that the corresponding DMRS is quasicollocated with its associated CRS with respect to delay spread, Dopplerspread, Doppler shift and average delay.

If DPS is assumed among 3 TPs in CoMP, at least two of the 3 TPs sharethe same DMRS VCID (virtual cell ID) according to the current LTEstandard. For example, TP1, TP2 and TP3 join 3TP CoMP, and TP1 and TP2may share VCID 100. In this case, if the NAICS UE, which receives DLservice from TP4, cancels interference from TP1 and TP2, the followingproblem occurs in respect of QCL.

The UE should discover CSI-RS or CRS, which is quasi collocated (QCL)with the corresponding DMRS, after discovering VCID 100 through BD.However, since VCID 100 is a value used by both TP1 and TP2, in case ofQCL related NA information received by the UE, VCID 100 is quasicollocated (QCL) with the CSI-RS or CRS of the TP1 and at the same timeis quasi collocated (QCL) with the CSI-RS or CRS of the TP2. Therefore,the UE cannot know a TP quasi collocated (QCL) with VCID 100.

To solve this ambiguousness, the UE according to the present inventionis not expected to cancel or suppress TM10 interference if VCID andnSCID are associated with multiple CSI-RSs or multiple CRSs.

Alternatively, in order that the base station previously prevents theabove information from being transmitted, the following configurationcan be made. The UE assumes that QCL transmission is performed only ifVCID and nSCID are associated with one CSI-RS or one CRS. Also, the basestation transmits QCL information for the NACIS UE only if VCID andnSCID are associated with one CSI-RS or one CRS.

In case of TM10 interference, QCL signaling is necessarily required forTM 10 interference cancellation of the NAICS UE. Therefore, if TM 10 isincluded in a TM set of network assistance (NA) information given to theUE, the base station should necessarily signal QCL information. To thisend, the following configuration can be made. The UE assumes that QCLtransmission is performed only if VCID and nSCID are associated with oneCSI-RS or one CRS and TM10 exists in a TM subset of NA information set.Also, the base station transmits QCL information for the NACIS UE onlyif VCID and nSCID are associated with one CSI-RS or one CRS and TM10exists in a TM subset of NA information set.

A feedback method according to one embodiment of the present inventionwill be described with reference to FIG. 16.

In step S161, the UE receives assistance information for cancelling aninterference signal transmitted from an interfering base station. Inthis case, the assistance information is the assistance information forcancelling interference, as described in the first to fifth embodiments,and its technical feature follows the aforementioned description.

In step S162, the UE cancels the interference signal on the basis of theassistance information, and receives a desired signal from the servingbase station. A detailed method for cancelling interference by using theassistance information follows the technical features described in thefirst to fifth embodiments. For example, the UE may assume some ofassistance information for cancelling an interference signal as arestricted value and receive the interference signal.

FIG. 17 is a diagram illustrating a base station and a user equipment,which may be applied to one embodiment of the present invention.

If a relay is included in a wireless communication system, communicationin a backhaul link is performed between the base station and the relayand communication in an access link is performed between the relay andthe user equipment. Accordingly, the base station or the user equipmentas shown may be replaced with the relay depending on the circumstances.

Referring to FIG. 17, the wireless communication system includes a basestation 1710 and a user equipment 1720. The base station 1710 includes aprocessor 1713, a memory 1714, and radio frequency (RF) units 1711 and1712. The processor 1713 may be configured to implement proceduresand/or methods suggested in the present invention. The memory 1714 isconnected with the processor 1713 and stores various kinds ofinformation related to the operation of the processor 1713. The RF unit1716 is connected with the processor 1713 and transmits and/or receivesa radio signal. The user equipment 1720 includes a processor 1723, amemory 1724, and radio frequency (RF) units 1721 and 1722. The processor1723 may be configured to implement procedures and/or methods suggestedin the present invention. The memory 1724 is connected with theprocessor 1723 and stores various kinds of information related to theoperation of the processor 1723. The RF units 1721 and 1722 areconnected with the processor 1723 and transmit and/or receive a radiosignal. The base station 1710 and/or the user equipment 1720 may have asingle antenna or multiple antennas.

The aforementioned embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment. Moreover, it will be apparent that someclaims referring to specific claims may be combined with another claimsreferring to the other claims other than the specific claims toconstitute the embodiment or add new claims by means of amendment afterthe application is filed.

A specific operation which has been herein described as being performedby the base station may be performed by an upper node of the basestation as the case may be. In other words, it will be apparent thatvarious operations performed for communication with the user equipmentin the network which includes a plurality of network nodes along withthe base station may be performed by the base station or network nodesother than the base station. The base station may be replaced withterminologies such as a fixed station, Node B, eNode B (eNB), and anaccess point (AP).

The embodiments according to the present invention may be implemented byvarious means, for example, hardware, firmware, software, or theircombination. If the embodiments according to the present invention areimplemented by hardware, the embodiments of the present invention may beimplemented by one or more application specific integrated circuits(ASICs), digital signal processors (DSPs), digital signal processingdevices (DSPDs), programmable logic devices (PLDs), field programmablegate arrays (FPGAs), processors, controllers, microcontrollers,microprocessors, etc.

If the embodiments according to the present invention are implemented byfirmware or software, the embodiment of the present invention may beimplemented by a type of a module, a procedure, or a function, whichperforms functions or operations described as above. A software code maybe stored in a memory unit and then may be driven by a processor.

The memory unit may be located inside or outside the processor totransmit and receive data to and from the processor through variousmeans which are well known.

It will be apparent to those skilled in the art that the presentinvention may be embodied in other specific forms without departing fromthe spirit and essential characteristics of the invention. Thus, theabove embodiments are to be considered in all respects as illustrativeand not restrictive. The scope of the invention should be determined byreasonable interpretation of the appended claims and all change whichcomes within the equivalent scope of the invention are included in thescope of the invention.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein. It is also obvious to those skilled in the art thatclaims that are not explicitly cited in each other in the appendedclaims may be presented in combination as an embodiment of the presentinvention or included as a new claim by a subsequent amendment after theapplication is filed.

INDUSTRIAL APPLICABILITY

The present invention may be used for wireless communication devicessuch as a user equipment, a relay and a base station.

1. A method for cancelling interference and receiving a signal by a userequipment in a wireless communication system, the method performed bythe user equipment comprising: receiving assistance information forcancelling an interference signal transmitted from an interfering basestation; cancelling the interference signal based on the assistanceinformation; and receiving a desired signal from a serving base station,wherein the user equipment assumes a part of the assistance informationfor cancelling the interference signal as a limited value and thenreceives the interference signal.
 2. The method of claim 1, wherein theuser equipment assumes that the number of data layers of theinterference signal is smaller than or equal to the number of datalayers of the desired signal.
 3. The method of claim 1, wherein the userequipment assumes that the number of data layers of the interferencesignal is smaller than or equal to a value obtained by subtracting thenumber of data layers of the desired signal from the number of receivingantennas of the user equipment.
 4. The method of claim 1, wherein theuser equipment assumes that a modulation order of the interferencesignal is smaller than or equal to a value obtained by subtracting thenumber of data layers of the desired signal from the number of receivingantennas of the user equipment.
 5. The method of claim 2, furthercomprising: calculating PMI of the interfering base station on the basisof the number of data layers of the assumed interference signal.
 6. Themethod of claim 1, further comprising: receiving whether the userequipment can calculate a starting symbol of an interference PDSCH(Physical Downlink Control Channel) through a PCFCH (Physical ControlFormat Indicator Channel) of the interfering base station, through RRC(Radio Resource Control) signaling.
 7. The method of claim 1, furthercomprising: receiving information on an interference DM-RS (demodulationreference signal), which may be connected with a CRS (Common ReferenceSignal) of the interfering base station through QCL (Quasi Co-location)A.
 8. A user equipment configured to cancel interference and receivedata in a wireless communication system, the user equipment comprising:a radio frequency (RF) unit; and a processor, wherein the processor isconfigured to receive assistance information for cancelling aninterference signal transmitted from an interfering base station, cancelthe interference signal on the basis of the assistance information, andreceive a desired signal from a serving base station, wherein theprocessor assumes a part of the assistance information for cancellingthe interference signal as a limited value, and then the user equipmentreceives the interference signal.
 9. The user equipment of claim 8,wherein the processor assumes that the number of data layers of theinterference signal is smaller than or equal to the number of datalayers of the desired signal.
 10. The user equipment of claim 8, whereinthe processor assumes that the number of data layers of the interferencesignal is smaller than or equal to a value obtained by subtracting thenumber of data layers of the desired signal from the number of receivingantennas of the user equipment.
 11. The user equipment of claim 8,wherein the processor assumes that a modulation order of theinterference signal is smaller than or equal to a value obtained bysubtracting the number of data layers of the desired signal from thenumber of receiving antennas of the user equipment.
 12. The userequipment of claim 9, wherein the processor is further configured tocalculate PMI of the interfering base station on the basis of the numberof data layers of the assumed interference signal.
 13. The userequipment of claim 8, wherein the processor is further configured toreceive whether a starting symbol of an interference PDSCH (PhysicalDownlink Control Channel) can be calculated through a PCFCH (PhysicalControl Format Indicator Channel) of the interfering base station,through RRC (Radio Resource Control) signaling.
 14. The user equipmentof claim 8, wherein the processor is further configured to receiveinformation on an interference DM-RS (demodulation reference signal),which may be connected with a CRS (Common Reference Signal) of theinterfering base station through QCL (Quasi Co-location) A.